The key to a healthy diet is to eat the right amount of calories for how active you are, so that you balance the energy you consume with the energy you use. If you eat or drink too much, you'll put on weight. If you eat and drink too little, you'll lose weight. You also need to eat a wide range of foods to ensure that you're getting a balanced diet and that your body is receiving all the nutrients it needs.
It is recommended that men have around 2,500 calories a day (10,500 kilojoules). Women should have around 2,000 calories a day (8,400 kilojoules). Most adults are eating more calories than they need, and should eat fewer calories.